Radiation and carcinogen-induced DNA damage and repair in human cancer-prone genetic diseases is being studied. Attention is presently focused on xeroderma pigmentosum (XP), a disease with ultraviolet sensitivity, and ataxia telangiectasia (AT), a disease which recently has been found to exhibit X-ray sensitivity. Cell fusion studies have demonstrated at least 5 different DNA repair defects among XP fibroblast strains. Similar studies to demonstrate possible genetic heterogeneity of DNA repair defects among AT strains are planned. A detailed comparison of the clinical features of these diseases is being made. Cell fusion studies are also planned to attempt to detect persons heterozygous for XP or AT. Correlation of clinical features of XP and AT with in vitro DNA repair defects may lead to futher understanding of the link between DNA repair and cancer. BIBLIOGRAPHIC REFERENCES: Robbins, J.H., Kraemer, K.H. and Andrews, A.D.: Inherited DNA repair defects in H. sapiens: Their relation to UV-associated processes in xeroderma pigmentosum. In Yuhas, J.M., Tennant, R.W. and Regan, J.D. (Eds.): Biology of Radiation Carcinogenesis. New York, Raven Press, 1976. pp. 115-127. Kraemer, K.H., Andrews, A.D., Barrett, S.F. and Robbins, J.H.: Colony forming ability of ultraviolet irradiated xeroderma pigmentosum fibroblasts from different DNA repair complementation groups. Biochim. Biophys. Acta 442: 147-153, 1976.